It is well known that chlorine-containing resins, particularly poly(vinyl chloride) polymers and copolymers, are unstable to heat and light and that the physical properties thereof are degraded upon exposure thereto. This degradation is typically manifested by development of or change in color. It is particularly noticeable in unstabilized polymers, i.e., polymers which do not contain stabilizers. Degradation or discoloration during processing is particularly undesirable in clear or lightly colored plastics. Therefore, it is desirable to prevent or inhibit the discoloration of plastics during processing so as to achieve useful products free of discoloration.
In order to minimize the discoloration and deterioration of various halogen-containing polymers such as vinyl chloride polymers and copolymers, various stabilizers such as lead-, cadmium-, and tin-based stabilizers have been developed and utilized. However, in recent years environmental pollution caused by the toxicity of the heavy metal residues and ecological considerations have stimulated further evaluation of such compounds and generated a search for alternative approaches.
Various compounds have been proposed for use in stabilizing halogen-containing polymers:
U.S. Pat. No. 3,928,285 to Gough et al. relates to a synergistic stabilizer composition comprising an organotin borate and an organic thiol.
U.S. Pat. No. 4,948,827 to Christidis relates to a thiophenol, prepared by reduction of tertiary butyl-4-toluenesulfonyl-2-chloride with the zinc-sulfuric acid couple, which reportedly can be used as a stabilizer for vinyl chloride polymers, as a chain-transfer agent, and as a peptizer.
European Patent Application No. EP 0 890 608 A2 relates to both flexible and rigid vinyl chloride polymer compositions incorporating a latent mercaptan-containing heat stabilizer which are reportedly substantially free from the offensive odor typically associated with mercaptans and are protected during processing by the degradation products of the latent (i.e., blocked) mercaptan, which include a free mercaptan. The free mercaptan thus released reportedly enhances the activity of metallic-based heat stabilizers such as zinc carboxylates and organotin carboxylates and mercaptides in the polymer composition. Other products of the degradation are believed to include carbocations of the blocking moiety which are stabilized by a molecular structure in which the electron deficiency is shared by several groups. The latent mercaptan comprises a 2-S-(tetrahydropyranyl)thioalkanol or a carboxylic acid ester thereof.
European Patent Application No. EP 0 945 484 A1 relates to compositions comprising halogen-containing polymers such as PVC resins which are reportedly stabilized against heat by a synergistic combination of a free mercaptan and a metal-based stabilizer and/or a Lewis acid such as zinc chloride.